The invention of this application relates to insulating laminates for electric use or metal clad laminates for producing electric printed circuit.
`Insulating laminate` in this application means laminate to be used as basic boards or supporting boards to mount electric components or parts, and `metal clad laminates` means boards for producing electric printed circuits to equip electric components. Conventional laminates are manufactured by impregnating varnish of thermosetting resin into a reinforcement such as phenol resin into paper, epoxy resin into paper or epoxy resin into glass cloth, producing prepregs from the resin impregnated reinforcement, laminating the prepregs or prepregs and metalic foil, for example electroytic copper foil on one or both sides of the laminated prepregs, and hardening them by pressing and heating.
Conventional insulating laminates for electric use, for example, insulating laminates containing phenol resin and paper thus manufactured as described above have a construction as illustrated in FIG. 3. Conventional metal clad laminates also usually have the similar cross section as that of FIG. 4. In the laminates, plural layers of paper impregnated with resin are through precise investigation, not isolated each other because of entanglement of fibers between each of the layers 1. It is well known that electric insulation, heat resistance and stability of dimension of insulating laminates for electric use and metal clad laminates for printed circuit boards are apt to be harmed when the laminates are exposed to moisture or absorb water, rendering unfavorable results for practical use. Usually water soaks into laminates from a surface layer, however, fibers of paper which constructs laminates and boundary area between fiber and resin facilitate soaking water.
Electric insulating resistance in the following two cases are compared in a humid atmosphere, (1) the surface of laminates as shown in FIG. 3 where paper fibers appear on the surface and, (2) the surface of laminates as shown in FIG. 4 where the surface thereof is covered with layer of adhesive for metal foil 5 or layer of the same resin which is impregnating the reinforcement 1. The electric resistance of the former laminates (1) exhibites remarkable decrease in comparison with that of the later laminates (2). The test teaches that fibers of paper contribute to decrease in insulating resistance, that is because of the main reason of water soaking into the surface layer and thence into the boards.
Laminates as shown in FIG. 4 are manufactured by process as disclosed in the Japanese unexamined patent specification SHO.48-31265. Character of the laminates as shown in FIG. 4 in a humid atmosphere is to a certain extent improved, soaking of water is temporarily prevented. However, laminates constructed in this manner do not settle the problem satisfactorily because resin layer is provided only at the surface, and water once passed through the resin layer easily soaks into the laminates. Therfore, once water enters in the surface layer, conventional laminates as illustrated in FIGS. 3 and 4 easily permit water to soak into the inner portion of laminates along continuous fibers extending from the surface to the inner portion of the laminates. Laminates having such construction as that of conventional laminates are apt to absorb moisture and water, which remarkably decreases electric insulation property, especially the ratio of electric volume resistance and the ratio of electric isolating resistance as defined by Japanese industrial standard JIS-C-6418. Absorbed moisture harms heat resistance to endure the heat of solder, and in the case of metal clad insulating laminates for a printed circuit, absorbed moisture in the reinforcement makes the laminates swell, lessens stability of dimension and have caused the laminates to warp.